It is well known in the art that electric current may be detected or measured by, for example, a current transducer. However, it is generally complicated, and often expensive, to make such a current transducer that will be able to withstand severe conditions such as, for example, a very high voltage environment under which the transducer may be used. The difficulties come mainly from the fact, among others, that materials used in a traditional current transducer are generally not capable of providing adequate electrical insulation for the transducer.
On the other hand, silica, being widely used in making optical fibers, is by nature a very good electrical insulator, or insulation material, under conditions of even extreme high voltages. Therefore, intuitively a current sensor made of optical fibers shall be able to operate under most, if not all, practically possible voltage conditions such as, for example, high voltage conditions where a traditional current transducer may not be able to operate and/or function properly. For this reason, along with considerations for other economic and technical benefits, current sensors based on fiber-optic have long been considered as good alternatives to the traditional current transducers.
A fiber-optic based current sensor works in the principle of the well-known Faraday effect. Specifically, a current propagating inside a wire or conductor generally will induce a magnetic field around the wire or conductor. Assuming an optical fiber is wound around the current-carrying wire or conductor, the magnetic field, through Faraday effect, may cause rotation of polarization direction of a light traveling inside the optical fiber. According to Faraday's law, the amount of rotation of polarization direction is directly proportional to the magnitude of electric current carried by the conductor or wire, as well as the total length of optical path traversed by the light. Therefore, by injecting a light with a pre-defined linear polarization state into a fiber situated in a sensing region (i.e., the magnetic field region) of the wire or conductor, and subsequently analyzing and/or measuring the polarization state of the light exiting from the sensing region, theoretically the amount of current carried by the wire or conductor may be determined. However, the magnitude of rotation of polarization direction is usually so small that detection of polarization rotation is seldom carried out in a direct fashion. Instead, a fiber-optic based current sensor usually employs a configuration of Sagnac interferometry for the detection of polarization rotation.
A Sagnac interferometry based fiber-optic current sensor operates under the principle of converting rotation of polarization state (or direction) of light into phase changes, that is, converting changes in a space domain into changes in a time domain, and detecting the phase changes using a Sagnac interference circuit. For example, a Sagnac interferometry based fiber-optic current sensor normally includes a fiber coil of one or more turns, and two quarter-wave plates attached to the input and output ports of the fiber coil. A linearly polarized light launched into a first quarter-wave plate may be converted into a circularly polarized light. While propagating through the fiber coil, the circularly polarized light may experience a rotation of polarization direction, in response to magnitude of a current being detected. The circularly polarized light may then be converted back into a linearly polarized light by a second quarter-wave plate with a phase shift.
As is well known in the art, in a conventional fiber-optic current sensor incorporating a two-by-two (2×2) optical coupler, an optical signal may be launched into an input port of the 2×2 optical coupler. The 2×2 optical coupler may split the optical signal into first and second linearly polarized lights. The two linearly polarized lights may subsequently become two circularly polarized lights, after propagating through two quarter-wave plates, and enter a fiber coil separately through the two ends or terminals of the fiber coil to propagate in opposite directions. After experiencing respective polarization rotation and exiting from the opposite end of the fiber coil, the first and second circularly polarized lights may be converted back into linearly polarized lights by the two quarter-wave plates. The two newly converted linearly polarized lights, carrying different phases corresponding to changes in polarization directions, may be launched into the 2×2 optical coupler for Sagnac interference. A resulting interfering signal may be used to estimate the magnitude of electric current under detection.
When an optical signal is launched into a 2×2 fiber coupler and splitted into two lights with one propagating along a throughput port and one propagating along a crossover port, a 90-degree phase difference is inherently added between the two output lights. In a traditional fiber-optic current sensor employing a 2×2 fiber coupler, in order to create effective coherent interference between two input lights, an additional phase shift needs to be added to the 2×2 fiber coupler to avoid its non-sensitive operating point of interference. This may be achieved by letting one of the input lights passing through a phase shift element. However, extra manufacturing complexity and additional device cost will be involved.
Conventional three-by-three (3×3) fiber couplers such as, for example, those made of conventional non-polarization maintaining fibers have also been used in the configuration of fiber current sensors. However, because a conventional 3×3 fiber coupler is not capable of properly maintaining polarization state of optical signal propagating inside, fiber-optic current sensors employing conventional 3×3 fiber couplers exhibit unstable performance due to, at least partially, uncertainty of the polarization state of light inside, and therefore have limited values in practice.
Therefore, there is a need in the art to develop solutions that will address above-mentioned shortcomings of present fiber-optic current sensors.